Root canals are surrounded by dentine and cementum that are insulators to electric current. At the minor apical foramen, however, there is a small hole in which conductive materials within the canal are electrically connected to the periodontal ligament that is a conductor of electric current. The resistive material of the canal (dentine, tissue, fluid) with a particular resistivity forms a resistor, the value of which depends on the length, cross-sectional area and the resistivity of the materials (Fig. 5). If an endodontic file penetrates inside the canal, and approaches the canal terminus, the resistance between the end of the instrument and the apical portion of the canal decreases, because the effective length of the resistive material inside the canal (l in Fig. 5) decreases.